The present invention relates to a process for preparing a tertiary amine-N-oxide, and more particularly to a process for preparing a tertiary amine-N-oxide in an improved yield which comprises reacting a tertiary amine with a peroxide in the presence of a specific catalyst.
When pyridine compounds or pyrazine compounds are used as a tertiary amine in the preparation of tertiary amine-N-oxides by the reaction with hydrogen peroxide, N-oxides of pyridine compounds or N-oxides of pyrazine compounds are prepared, which are a useful compound as an intermediate for the synthesis of medicinal and agricultural compounds.
Hitherto, processes for preparing a tertiary amine-N-oxide as exemplified below are known.
As to a process for preparing N-oxides of pyrazine compounds, there are known (i) a process for preparing N-oxides of pyrazine compounds by reacting pyrazine compounds with hydrogen peroxide in acetic acid (J. Org. Chem., 23, 1603(1958)), (ii) a process for preparing N-oxides of pyrazine compounds by reacting pyrazine compounds with hydrogen peroxide in water as a solvent, using sodium tungstate as a catalyst (Chem. Pharm. Bull., 22, 2097(1974)), and (iii) a process for preparing N-oxides of pyrazine compounds by reacting pyrazine compounds with hydrogen peroxide in water as a solvent using molybdenum trioxide as a catalyst (Japanese Unexamined Patent Publication No. 181268/1987).
However, these known processes have various defects as mentioned below. In the process (i), it is necessary to use a lot of acetic acid as a solvent and, therefore, complicated steps are required to separate and recover the produced N-oxides of pyrazine compounds and acetic acid. In the process (ii), the yield of the desired compound is as low as 47%.
On the other hand, in the process (iii), the desired compound is obtained in a high yield. However, in the process (iii), there is a problem as well as in the process (ii), that is, it is difficult to easily separate the catalyst from the reaction mixture by means of the simple procedure like filtration, because the catalyst is dissolved in considerable proportion in the reaction mixture. This residual catalyst in the reaction mixture results in the contamination to the desired product separated by use of the conventional separation procedure such as removal of the solvent from the reaction mixture by distillation and subsequent crystallation from the resultant residue.
An object of the invention is to provide a process for preparing a tertiary amine-N-oxide whereby the above-mentioned problems in the known processes can be solved.
A further object of the present invention is to provide a process for preparing a tertiary amine-N-oxide in a high yield and in a high purity.
These and other objects of the present invention will become apparent from the description hereinafter.